1. Field of the Invention
The present invention relates to an image signal processing apparatus for processing an output image signal of an image scanner which electrically scans a text image.
2. Description of the Prior Art
When a density of a text image is electrically picked up by a solid-state image pickup device such as a charge coupled device (CCD) and the electric signal is transmitted in the form of a black-white binary signal for printing, the quality of a reproduced image is poor because of the affect of shading included in the image pickup device and any optical system. When the image is picked up by the solid-state image pickup device such as a CCD, an output image signal waveform from the solid-state image pickup device is not necessarily flat even if the text image is entirely white. This phenomenon is called shading. One of major causes of the shading is that the transfer efficiency of each of the cells of the solid-state image pickup device such as a CCD is not 100%. Other causes of the shading are manufacturing variance of the solid-stage image pickup device such as a CCD, distortion at a periphery of an optical lens inserted between the text and the CCD and nonuniformity of a light source for illuminating the text.
In order to resolve the above problem, approaches have been proposed in Japanese Published Unexamined Patent Application Nos. 53-139421 and 54-21219 in which the shading is reduced by a relatively simple circuit including a low-pass filter and a delay line. However, this method is not applicable when a half-tone is to be reproduced using a systematic dizzer method. Furthermore, high fidelity reproduction and high quality of print cannot be expected unless the level, the gain and the linearity of the image signal are exactly matched to those of the original text.
On contrary, if the high fidelity reproduction is attained, a desired image may not be sharp because a background of the text is reproduced.
In addition, noise generated during the processing of the image signal or noise images such as smears in the background may be reproduced or emphasized resulting in the degradation of the quality of the reproduced image.
A process for emphasizing an outline of the read image is known. When this process is used, small noise in the background or noise generated in the processing circuit is emphasized resulting in a poor quality of the reproduced image.
When a line scan type photoelectric image pickup device which reads the text in a main scan direction is used and the image pickup device or an optical system is mechanically moved in a sub-scan direction relative to the text to scan the entire image on the text, if the outline emphasis process is carried out in both the main scan direction and the sub-scan direction, the image reproduced from the image signal read from the 90-degree rotated text and the image reproduced from the image signal read from the non-rotated text are different because of a difference between the resolving power in the main scan direction and the resolving power in the sub-scan direction.
When a plurality of CCD's are arranged serially along the line to read a wide width text, the characteristics of the processing circuits must be matched to the characteristics of the respective CCD's.
On the other hand, when the image signal is converted to a digital signal and is processed for the outline emphasis, the outline emphasis circuit for the outline emphasis process needs a large number of components. When such outline emphasis circuits are used, one for each of the image pickup devices, the cost is increased and the size of the apparatus is increased.
When the text image is read by the image pickup device such as a CCD through an optical system to produce the image signal, an MTF of a lens of the optical system and a spatial frequency characteristic of the CCD including the lens are not constant. For example, CCD image pickup devices have large manufacturing variance and do not have a constant spatial frequency characteristic. When the light quantity is to be controlled by an iris, the MTF is lowered as the aperture of the iris increases.